For me, one of the great challenges in electronics is to see just how small a robot I can make. The beautiful thing about electronics is that the components just keep getting smaller and cheaper and more efficient at an incredibly fast pace. Imagine if automobile technology were like that. Unfortunately, mechanical systems at this time, are not advancing nearly as fast as electronics.
This leads to one of the main difficulties in building very small robots: trying to fit in a small space, the mechanical system that moves the robot. The mechanical system and batteries tend to take up most of the volume of a really small robot.
pic1 shows Mr. Cube R-16, a one cubic inch micro-sumo robot that is capable of reacting to its environment with music wire whiskers (bumper switch). It can move and explore the perimeter of a small box. It can be remote controlled using a universal TV infrared remote control that is set up for a Sony TV. It can also have its Picaxe microcontroller pre-programmed with reaction patterns. Details begin on step 1.